1. Field of the Invention
The present invention relates to techniques for performing formation evaluation of a subterranean formation by a down hole tool positioned in a well bore penetrating the subterranean formation. More particularly, but not by way of limitation, the present invention relates to techniques for making measurements of formation fluids.
2. Background of the Related Art
Well bores are drilled to locate and produce hydrocarbons. A down hole drilling tool with a bit at an end thereof is advanced into the ground to form a well bore. As the drilling tool is advanced, a drilling mud is pumped through the drilling tool and out the drill bit to cool the drilling tool and carry away cuttings. The drilling mud additionally forms a mud cake that lines the well bore.
During the drilling operation, it is desirable to perform various evaluations of the formations penetrated by the well bore. In some cases, the drilling tool may be removed and a wire line tool may be deployed into the well bore to test and/or sample the formation. In other cases, the drilling tool may be provided with devices to test and/or sample the surrounding formation and the drilling tool may be used to perform the testing or sampling. These samples or tests may be used, for example, to locate valuable hydrocarbons.
Formation evaluation often requires that fluid from the formation be drawn into the down hole tool for testing and/or sampling. Various devices, such as probes, are extended from the down hole tool to establish fluid communication with the formation surrounding the well bore and to draw fluid into the down hole tool. A typical probe is a circular element extended from the down hole tool and positioned against the sidewall of the well bore. A rubber packer at the end of the probe is used to create a seal with the wall of the well bore. Another device used to form a seal with the well bore is referred to as a dual packer. With a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the well bore there between. The rings form a seal with the well bore wall and permit fluid to be drawn into the isolated portion of the well bore and into an inlet in the down hole tool.
The mud cake lining the well bore is often useful in assisting the probe and/or dual packers in making the seal with the well bore wall. Once the seal is made, fluid from the formation is drawn into the down hole tool through an inlet by lowering the pressure in the down hole tool. Examples of probes and/or packers used in down hole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568 and 6,719,049 and U.S. patent application Ser. No. 2004/0000433.
Formation evaluation is typically performed on fluids drawn into the down hole tool. Techniques currently exist for performing various measurements, pretests and/or sample collection of fluids that enter the down hole tool.
Fluid passing through the down hole tool may be tested to determine various down hole parameters or properties. Various properties of hydrocarbon reservoir fluids, such as viscosity, density and phase behavior of the fluid at reservoir conditions, may be used to evaluate potential reserves, determine flow in porous media and design completion, separation, treating, and metering systems, among others.
Additionally, samples of the fluid may be collected in the down hole tool and retrieved at the surface. The down hole tool stores the formation fluid in one or more sample chambers or bottles and retrieves the bottles to the surface while keeping the formation fluid pressurized. An example of this type of sampling is described in U.S. Pat. No. 6688390. Such samples are sometimes referred to as live-fluids. These fluids may then be sent to an appropriate laboratory for further analysis. Typical fluid analysis or characterization may include, for example, composition analysis, fluid properties and phase behavior. In some cases, such analysis may also be made at the well site surface using a transportable lab system.
Techniques have been developed to perform surface testing of the live-fluids. Many fluid measurements can require on the order of an hour or more time. For example, with phase behavior analysis or determination, the fluid begins as a single phase, liquid or gas. The temperature is held constant. The volume is expanded in a series of small steps. Before the next step in volume is taken, the pressure must be stable. In order to accelerate the time required to stabilize the pressure, the fluid is actively mixed. Such mixing typically involves stirring, churning, shearing, vibrating and/or otherwise transporting the fluid volume. During the volume expansion process or steps, optical technologies are used to detect the presence of a separate phase. For example, a 2 micron resolution high pressure camera may be used to take pictures, via an optical window, and a measurement of light absorbance may be made using Near Infra Red (NIR).
During sampling, reservoir fluid may exhibit a variety of phase transitions. Often these transitions are the result of cooling, pressure depletion and/or compositional changes that occur as the fluid is drawn into the tool and/or retrieved to the surface. The characterization of fluid phase behavior is key to the planning and optimization of field development and production. Changes of temperature (T) and pressure (P) of the formation fluid often lead to multi-phase separation (e.g., liquid-vapor, liquid-solid, liquid-liquid, vapor-liquid, etc.), and phase recombination. Similarly, a single-phase gas typically has an envelope at which a liquid phase separates, known as the dew point. These changes can affect the measurements taken during formation evaluation. Moreover, there is a significant delay in time between sampling and testing at the surface or offsite laboratories.
It is, therefore, desirable to provide techniques capable of performing formation evaluation of fluid that is representative of fluid in the formation. It is further desirable that such techniques provide accurate and real-time measurements. Such formation evaluation would need to operate within size and time constraints of well bore operations, and preferably are performed down hole. It is to such a fluid analysis assembly capable of effecting such formation evaluation that the present invention is directed.